1. Field of the Invention
Embodiments of the invention relate to DC-DC converters and, more particularly, to multilevel topologies for such converters.
2. Description of the Related Art
DC-DC converters are a class of power converter. They are used to convert a direct current (DC) signal from one voltage level to another. These converters are commonly used in portable electronic devices that are powered by batteries, such as laptops and cellular phones. DC-DC converters are particularly useful in applications that have several different sub-systems, requiring several different voltage input levels.
There are several different schemes for DC-DC conversion. Linear regulators convert an input voltage to a lower output voltage by dissipating power through thermal radiation. For larger voltage drop high-current applications, these devices are inefficient and, thus, rarely used. A more commonly used scheme is switched-mode conversion. Switch-mode converters convert voltages by periodically storing energy in inductive and/or capacitive components and then releasing that energy to produce the desired voltage level. Inductive components store energy in the form of a magnetic field, whereas capacitive components store energy in an electric field.
DC-DC converters that use a magnetic energy storage mechanism employ inductors or transformers. The output voltage is controlled by modulating the duty cycle of the voltages used to charge the inductive component. Common types of magnetic storage DC-DC converters include buck and boost converters.
FIG. 1a and FIG. 1b are circuit diagrams of a typical boost converter 10. Energy is periodically stored in an inductor L and then released to the load. During each periodic cycle, a switch S is used to allow current to flow through the inductor. When the switch is closed, current flows through the inductor and stores energy from the current in a magnetic field. During this time, the switch acts like a short circuit in parallel with the diode and the load, so no inductor current flows to the load. When the switch is opened, the short circuit is removed and inductor current is allowed to flow through the load; this increases the impedance of the circuit, which requires either a decrease in current or an increase in voltage to maintain a constant output voltage. The inductor will tend to resist such a sudden change in the current, which it does by acting as a voltage source in series with the input source, thus increasing the total voltage seen by the load and thereby preserving (for a brief moment) the current level that was seen when the switch was closed. This is done using the energy stored by the inductor. Over time, the energy stored in the inductor will discharge into the load, bringing the net voltage back down. If the switch is cycled fast enough, the inductor will not discharge fully in between charging stages, and the load will always see a voltage greater than that of the input source alone when the switch is opened.
FIG. 2a and FIG. 2b are circuit diagrams of a typical buck converter 20. Energy is periodically stored in an inductor L and then released to the load. During each periodic cycle, two switches SWH and SWL are used to alternately connect one end of inductor L to input source VIN during the charge phase and to ground during the discharge phase. When the high side switch SWH is closed (shown in FIG. 2a), current through the inductor L (IL) rises linearly, charging the inductor L. Then SWH is opened and the low side switch SWL is closed (shown in FIG. 2b), and IL decreases linearly, discharging the inductor into the load. As the inductor L is discharging, IL decreases but still flows in the same direction into the load because the stored magnetic energy prevents the current through the inductor from changing direction instantaneously. The switches are turned on and off periodically at a fixed frequency such that the duty cycle determines the ratio of output voltage to input voltage. If the high side switch is opened before the inductor is fully charged, there will always be a voltage drop across the inductor, such that the net voltage seen by the load will always be less than the input voltage source.
At least one challenge associated with boost and buck converters are a reduced efficiency at high switching frequencies, as well as power loss. In some applications, for example wireless applications, in order to increase the power efficiency, the converter providing the supply voltage can be modulated using “envelope tracking,” wherein the converter is arranged such that its output voltage tracks an envelope signal. However, converters using envelope tracking need to change the output voltage in short time periods, such as a matter of a few nanoseconds. During the short time periods, the converters have to charge and discharge filter capacitors in the range of several microfarads. This fast charging/discharging calls for high frequency and high power converters, which can be bulky and inefficient.